Well, that depends on how you approach and research a topic, not on how old something is. Archaeology has no cut-off point of age when conducting research. You can research an area that is only a few weeks old as Zimmerman et al (2010) did in their research on homelessness. Archaeologists also conduct ethnoarchaeology and research on contemporary people and their practices in order to understand the archaeological record. So it is not an age issue, but a methodology issue. How you form you research question and try to answer that question dictates whether something is archaeology, history, etc. And of course, there is always a little blurring of the lines between fields when you approach a particular topic that is researched by multiple fields.